Low energy electron microscopy (LEEM) and photo electron emission microscopy (PEEM) are both examples of cathode lens microscopy, in which a strong electric field is maintained between a sample under study and an objective lens of a microscope. In such instruments, the sample is considered the cathode and the objective lens is considered the anode. Electrons are reflected from the sample in the case of a LEEM instrument, or photo-emitted by the sample in case of a PEEM instrument, at low energy, for example, less than 500 eV. The electrons are accelerated into the objective lens, reaching an energy of 10-30 keV. Subsequently, these electrons are utilized to form an image of the sample on a viewing screen.
The backfocal plane of the objective lens of the microscope provides an image of the angular distribution of the electrons, which contains information on the arrangement of the atoms in the outer layers of the sample. This image is considered a low energy electron diffraction (LEED) pattern for LEEM, or a photo electron diffraction (PED) pattern for PEEM. The energy distribution of these electrons may also contain information about the electronic and chemical nature of the surface under study.
Energy filtering of the electrons allows an operator to view an image of the sample at a specified electron energy corresponding to, for example, the binding energy of electrons of a particular chemical element. Alternatively, by operating projector and spectrometer lenses of the microscope at a different excitation, the energy filtered PED pattern may be observed. The combination of an energy filtering cathode lens microscopy instrument with synchrotron radiation provides the operator with an extremely powerful analytical tool in the study of surface and interface structure and composition.
A schematic view of a typical energy filtering combination LEEM/PEEM instrument is provided in FIG. 1. In an LEEM instrument, an electron gun generates an electron beam 102 at, for example, 15 keV electron energy. Condenser lenses 104 focus electron beam 102. A magnetic field in a magnetic deflector 106 deflects electron beam 102 over a large angle, for example, 60 degrees. This deflection directs electron beam 102 into an objective lens 108 and to a sample 110. After a reflection of electron beam 102 from sample 110, the electrons retrace their path, forming a LEED pattern in a backfocal plane 112 of objective lens 108, and a real space image of the sample in a center of magnetic deflector 106. The electrons are then deflected into a projector column 114 of the microscope.
Alternatively, in a PEEM instrument, sample 110 is illuminated with ultra violet (UV) light or soft x-ray photons 117 to generate photo electrons from sample 110. In this embodiment the electron gun is not utilized. Backfocal plane 112 contains a PED pattern, and a real space image of the sample is again formed in the center of magnetic deflector 106.
Projector column 114 contains lenses 116 for magnification of the image or the diffraction pattern onto a viewing screen 118. Additionally, projector column 114 contains an electrostatic or magnetic electron spectrometer 120 together with necessary coupling lenses 122 to energy filter the electrons. Coupling lenses 122 serve to select either the image or the diffraction pattern for energy filtering. If the image is to be filtered, the diffraction pattern is located at a spectrometer entrance aperture 124. Spectrometer 120 focuses the diffraction pattern onto an exit aperture 126. Different electron energies are deflected over different angles in spectrometer 120, and dispersed across exit aperture 126. Thus, exit aperture 126 selects electrons over a narrow energy range, corresponding to the width of the aperture, blocking electrons outside this range. Projector lenses 116 on the exit side of spectrometer 120 serve to magnify the energy filtered real space image and project it onto viewing screen 118.
To obtain an energy filtered diffraction pattern, spectrometer coupling lenses 122 focus the real space image on spectrometer entrance aperture 124. Spectrometer 120 again focuses this image onto exit aperture 126, while dispersing the electrons according to their energy. Exit aperture 126 selects a narrow energy range, and the projector lenses on the exit side of spectrometer 120 now magnify the diffraction pattern onto viewing screen 118.
To change from an energy filtered image to an energy filtered diffraction pattern, a large change in the excitations and focal lengths of spectrometer coupling lenses 122 is required on both the entrance and exit sides of spectrometer 120. In some cases, this may add up to six electron lenses to the microscopy instrument. Spectrometer 120 is a relatively complex piece of hardware, and the setup of the instrument is of great complexity and cost.